Portable electronic devices such as laptop PCs, tablet PCs, digital cameras, mobile phones, portable gaming devices, earphone-type music players, wireless headsets, hearing aids, recorders, which are portable while being used by the user are rapidly increasing in recent years. Many of these portable electronic devices have therein a rechargeable battery, which requires periodical charging. To facilitate the work for charging the rechargeable battery of an electronic device, there are an increasing number of devices for charging rechargeable batteries by using a power supplying technology (wireless power transmission technology performing power transmission by varying the magnetic field) that performs wireless power transmission between a power-supplying device and a power-receiving device mounted in an electronic device.
For Example, as a wireless power transmission technology, there have been known, for Example, a technology that performs power transmission by means of electromagnetic induction between coils (e.g. see PTL 1), a technology that performs power transmission by means of resonance phenomenon (magnetic field resonant state) between resonators (coils) provided to the power-supplying device and the power-receiving device (e.g. see PTL 2).
As a method for charging a rechargeable battery (e.g., a lithium ion secondary battery), a constant current-constant voltage charging system has been known. In the constant current-constant voltage charging system, charging is performed by a constant current for a while after the start of the charging. When the voltage applied while the charging by the constant current is being performed increases to a predetermined upper limit voltage, the charging is performed by a constant voltage while the voltage is maintained at the upper limit voltage.